Project Summary Animals must cope with the pervasive force of gravity as they navigate the environment. To sense and respond to this force, vertebrates rely on signals from the inner ear, where gravito-inertial sensors called otoliths drive activity in peripheral vestibular circuits. This information is then processed by central vestibular neurons in the brainstem and transformed into postural outputs via projections to the spinal cord. Although studies in slice preparation have indicated that vestibular neurons make linear computations of their inputs, this concept has not been tested in vivo. The objective of this proposal is to determine how vestibulospinal neurons carry out computations of sensory inputs. To surmount the technical challenges of examining synaptic and cellular properties of this circuit, I propose to use the larval zebrafish. Zebrafish are an excellent system for this line of research because of their accessibility, transparency, and homology to other vertebrates. Furthermore, we can carry out many experiments that are not feasible in mammalian models, including in vivo whole cell patch- clamp analysis of synaptic responses to sensory stimuli. This technical advance permits us to record sensory- evoked activity in the intact brain, over the time period in which postural behaviors develop. In addition, we can exploit a mutant fish line in which otolith development is delayed by two weeks, providing in effect a high selective sensory deprivation to vestibular circuits. The proposed experiments will therefore reveal how sensory information is encoded during development, both under normal conditions and those of sensory delay. In Aim 1, we will use a combination of behavior, imaging, and physiology to define the anatomy, sensory responses, and functional role of vestibulospinal neurons in vivo. These experiments will define the homology between zebrafish and mammalian vestibulospinal nuclei. In Aim 2, we will quantify how sensory afferents converge to produce central tuning. We will further ask how this convergence develops over the time period in which animals begin to self-right with respect to gravity. Here we will use both ultrastructural reconstructions of vestibular afferents to the central vestibulospinal neurons as well as physiological analyses of the development of sensory encoding. Finally, in Aim 3 we will examine the functional contributions of inhibition to sensory tuning and develop a highly constrained model of vestibular computations. Together, the proposed experiments will provide a rigorous and quantitative analysis of how sensory tuning is constructed in central vestibular neurons.